1. Field of the Invention
The invention relates to a water hydraulic screw prestressing device free of torque with integrated ultrasonic prestressing monitoring.
2. Discussion of Background Information
Threaded assemblies provide detachable connections of components. The important factor for these connections is the spring action of the screw and screwed parts (flange). Tightening a screw with a prestressing force causes a stretching of the screw and a compression of the flange. The resulting friction produces a connection of the two parts in the radial direction, where the prestressing force itself blocks the axial degree of freedom of the parts involved.
The most common tightening methods, i.e., methods of applying a prestressing, can be divided into two groups. With the first group, the shanks of the screws are strained by the nuts being rotated. The screws, or studs, are stretched and strained by the rotary motion according to the pitch of the thread. These standard threaded assemblies have the disadvantage that if the maximum prestressing forces are utilized in the screw, or stud, torques are inevitably transferred via the unavoidable thread friction. This leads to a superimposition of the stress in the screw, or stud, (tensile stress plus shearing stress) and furthermore, to a transfer of the thread torque to the flange to be tightened. Due to the superimposition of the stress in the screw, or stud (tensile stress plus shearing stress), the threaded assembly cannot be tightened up to the maximum permissible tensile stress. Furthermore, in the case of delicate parts, such as, e.g., the ceramic mirrors of a spectrometer (e.g., in the NirSpec space project), the transfer of the thread torque to the flange to be tightened leads to an unacceptable and inadmissible deformation of the mirror surface.
In the NirSpec space project (the follow-up project to the Hubble telescope) and, as expected in all future optical space projects, all optical mirrors (e.g., ceramic) have to be tightened via frictional contact and a three-screw attachment with the highest prestressing force and, as far as possible, free of torque. In-house tests with conventional tightening methods have shown that torques transferred to the ceramic mirror foot have a negative impact (i.e., performance loss) on the high demands on the mirror surface.
With the second group of tightening methods, the shanks of the screws are first stretched (e.g., with the aid of hydraulic cylinders), then the nuts are applied in the stretched state of the screws, or studs. After the stretching force is removed, the connection is braced. Torsional friction effects are thus of only secondary importance; shearing stresses in the studs virtually do not occur. In the prior art, hydraulic screw prestressing units are known from the construction of nuclear power plants and wind turbine generator systems. However, these units are known only for very large threaded assemblies (from diameter M24 upwards) and all operate on the basis of oil hydraulics. A hydraulic screw prestressing device for threaded assemblies from M4 to M12 is not known in the prior art. Additionally, water hydraulic screw prestressing devices are not known in the prior art.
Through the use of an oil hydraulic system, these screw prestressing devices have the disadvantage that in practice they are not suitable for cleanroom applications, such as are necessary, e.g., in applications in the field of space flight. An oil hydraulic system can be used in a cleanroom only to a limited extent, namely with special, complex additional measures.
The known hydraulic screw prestressing units, which are known only for very large threaded assemblies, furthermore have the disadvantage that, in the case of damage, they would lead to a destruction of sensitive elements, e.g., highly sensitive optical elements, such as are used in the field of space flight.
In space flight projects, most reduced-weight threaded assemblies have to be monitored and tightened with the highest prestressing forces. In particular, with the use of so-called optical benches, threaded assemblies are to be tightened if possible without the influence of torque, while at the same time with the highest cleanness class. Every friction produced (e.g., metal on metal, but also metal on ceramic) inevitably leads to abrasive wear, which in turn can impact the optical surfaces. The subject matter of the invention solves these problems in particular in an optimal manner.